Present board assembly processes using reflow soldering require the use of solder paste to provide the solder necessary to form solder joints between components and the printed circuit board. Solder paste is expensive, and a significant cost advantage could be accrued by eliminating it from a process if a thick coating of solder covered the underlying copper conductors. One method of providing a solder coating is dipping the printed circuit board in molten solder, but this method produces coatings that are thin in some areas and thick in others. The thin areas form an intermetallic compound of copper and tin on exposure to the oxidizing environment of the atmosphere, thereby rendering the printed circuit board unsolderable. The thick solder deposits on some of the solder pads become rounded due to the surface tension of the solder. These rounded or domed surfaces create significant problems when leadless components are placed manually or automatically. In addition, the uneven coating provided by the dipping process creates solder pads of varying heights, another undesirable feature when employing leadless components.
Methods of providing a thick solder coating using plating methods have been documented in the literature. For example, "Fluxless SMD Soldering" by Hendricks and lnpyn of General Electric Company in Circuits Manufacturing, Oct. 1984, details the use of heavy solder plating to assemble printed circuit boards and leadless components. Referring to FIG. 1, plating a printed circuit board 10 with a heavy coating of solder 12 is a preferred method to pre-apply solder on printed circuit boards because the plated solder surface is flat, a desirable condition when placing leadless components. Typically, solder flux 13 is applied to the solder 12 to remove oxides and to aid in reflowing the solder. However as seen in FIG. 2, this advantage of flat solder pads is lost for double sided boards because the solder 16 on one side domes up when the printed circuit board is heated to reflow the components 14 on the other side. When boards are plated with heavy coatings of solder, the reflow operation solders the components 14 to the board and insures adequate alloying of the solder 18 to the underlying copper 19. This reflowing procedure also causes the solder 16 to wet to the solder pads on those pads that do not contain components, such as those on the second side, creating a domed solder pad 30 which makes the automatic or manual placement of leadless components 15 difficult, due to the rounded nature of the top of the solder pad 20 (FIG. 3). Leadless components do not remain in place on the round surface, and become skewed and misaligned, resulting in solder defects.
Clearly, these methods of eliminating solder paste are not suitable for use with double sided printed circuit boards having surface mounted components, where components are soldered to the printed circuit board in two steps. An improved method of assembling double sided printed circuit assemblies that eliminates solder paste and provides flat solder pads for leadless components is certainly desirable. As a result, a need exists for an improved method of soldering double sided printed circuit boards that are pre-clad with solder.